The use of dichroic dyes in solution with various liquid crystal materials, and liquid crystal display devices incorporating such mixtures are well-known in the art. These dichroic dye-liquid crystal mixtures may be used in "guest-host" type liquid crystal displays or, when an optically-active additive is employed, in "phase-change" type liquid crystal displays. Additionally, dichroic dyes may be utilized in smectic liquid crystal display devices as disclosed in U.S. Pat. No. 4,196,974, and commonly assigned U.S. Ser. No. 251,247, filed Apr. 6, 1981, now U.S. Pat. No. 4,391,492, and U.S. Ser. No. 436,030, filed Oct. 22, 1982.
Many of these dichroic dyes are elongated molecules which exhibit little or no absorption of light vibrating parallel to the long axis, while absorbing that light in various portions of the visible spectrum which vibrates perpendicular to the long axis. Other dichroic dyes may align oppositely, being colorless (non-absorbing) with respect to light vibrating parallel to the short axis of the molecule and colored (absorbing) with respect to light vibrating perpendicular to the short axis.
In the above-mentioned types of displays the host liquid crystal material has its orientation controlled by the application of electric fields and in turn interacts with the guest dichroic dye to induce a cooperative conforming alignment. The dichroic dye manifests low or high absorbance of incident light depending upon its molecular orientation with respect to the light wave vector. Thus, generally when an electric field is applied to the display, the area between the electrodes appears clear, i.e., all the dyes exhibit minimum absorption, and in the absence of an electric field, the display appears characteristically dye colored, i.e., the dyes are in a high absorbing state.
It is known that by combining two different dichroic dyes, one which absorbs light parallel to the short axis of the molecule, and one which absorbs light parallel to the long axis of the molecule, devices which change from one color to another are possible. It has also been disclosed in U.S. Pat. Nos. 4,105,299 and 4,154,746 that an isotropic, i.e., nondichroic dyestuff, could be combined with a dichroic dyestuff so that the colors of the two would be additive at one stage, and the color of the isotropic dyestuff would appear at the other stage of the electric field cycle, so that a device which changes from one color to another would be possible. One difficulty in providing two-color displays by utilizing isotropic dyes has been to find dyes which are not only isotropic, but which are non-ionic, have acceptable extinction coefficients, sufficient solubility in liquid crystal materials, and good photostability.